This invention relates to a hydraulic damper and, particularly to a hydraulic damper of the kind including generally vertically disposed coaxial inner and outer tubes, a piston working in the inner tube and partitioning the interior thereof into first and second liquid chambers, a piston rod secured to the piston and extending through the first liquid chamber to the outside of the damper through upper ends of the inner and outer tubes, a rod guide supporting the upper ends of the inner and outer tubes and slidably guiding the piston rod, a base cap supporting the lower ends of the inner and outer tubes, and a reservoir chamber defined between the inner and outer tubes and containing therein liquid and gas.
In a hydraulic damper of the kind aforementioned, the damping function is performed by forcibly flowing the liquid through a restricted passage. For obtaining desired damping forces both in the extension and contraction strokes of the piston rod, a high pressure gas is usually enclosed in the reservoir chamber. However, the pressure of the gas enclosed in the damper may sometimes decrease during long period of usage. Thus, it has been requested to provide a countermeasure to compensate for the decrease in the pressure of the gas enlcosed in the damper.
One prior art hydraulic damper of the kind aforementioned comprises a bottom valve mechanism mounted on the inner end of the inner tube to control liquid flow between the second chamber and the reservoir chamber, and the bottom valve consists of a resilient annular disc with the inner and outer circumferences engaging respectively with opposingly facing annular valve seats. In the contraction stroke of the piston rod, the inner circumference of the disc separates from one valve seat to allow liquid flow from the second chamber to the reservoir and, in the extension stroke of the damper, the outer circumference of the disc separates from another corresponding valve seat to permit liquid flow from the reservoir to the second chamber. The damper works satisfactorily, however, there is a shortcoming that the function of the bottom valve depends solely on the resiliency of the disc thereby complicating fabrication and adjustment procedures during mass production.
It is also known to form the bottom valve mechanism of two separate valve members for the extension and contraction strokes. When either one of the two valve members opens, the other valve member closes. The function of the bottom valve mechanism can very easily be adjusted, but it is complicated in construction, and is expensive.